1. Field of the Invention
The invention relates to an active coupler between an optical bus line and subscribers, and more particularly its subject is a coupler of this type for an optical bus line, transmitting data in digital form, as well as a bus line equipped with such active couplers.
The term optical bus line issused to designate a line for transmission of data by optical conductors, serving a certain number of subscribers in parallel by means of couplers, thanks to which each of the subscribers is able both to receive data from a source or from other subscribers, and to transmit toward this source or toward other subscribers. In the case of a monodirectional coupler, a subscriber can receive data only from the source or from subscribers upline, situated between the source and himself on the bus line, and can transmit only to downline subscribers, while in the case of the two-directional coupler, each subscriber can receive data from any other subscriber, and can transmit for his own purpose, whatever his position on the bus line is. The optical conductors used to constitute the line can be monofilament or multifilament conductors.
2. Description of the Prior Art
Optical bus lines are already known, on which, at the level of each subscriber, an optical coupler insures the coupling of an optical transmitting conductor with an optical receiving conductor, that receives a transmitted fraction of the incident light energy transmitted by the transmitting conductor, the optical coupler being equipped with a device for diverting a fraction of the incident light energy, in order to insure the reception by the subscriber of an output signal constituted by, or proportional to this diverted fraction. The diversion device is constituted, for example, by a semireflecting plate allowing a fraction of the light energy, transmitted from the transmitting conductor to pass the receiving conductor, and insuring, by reflection, the diversion of a fraction of the incident light energy. When the subscriber transmits a signal, the latter can be injected, in the form of light energy, into one of the optical conductors, and will thus be transmitted to the other subscribers. Reverting to the preceding example, an input light signal can be injected into the optical receiving conductor by the semireflecting plate. These known optical bus lines have the drawback that the diversion of a fraction of the incident energy at the level of each coupler, makes for a loss of power of the optical signal circulating in the line. Since the insertion losses of an optical coupler are, for example, on the order of 3 decibels (db), it will be easy to imagine the difficulties arising on the reception at the level of the tenth consecutive subscriber on the line, which will receive the data at a level 30 db lower than that at which the first subscriber receives the data.
To relieve this drawback, there have already been proposals to equip the bus line at the level of each subscriber with a repeater interrupting the optical path, and constituted by a photoreceiver situated at the end of the upline optical conductor, and emitting an output signal intended for the subscriber, this signal, at the same time, being correctly amplified, then sent to the terminals of a phototransmitter which reinjects into the downline optical conductor, a light signal whose level has been restored, without occasioning a loss of power at the level of the subscriber. But these bus lines with repeaters have another drawback, which is that any transmission is made impossible in the event of a breakdown of a single repeater.